Safety stops: hardwired or integrated?

Latest-generation drive technologies with integrated safety-stop functionality offer a viable alternative to the traditional hardwired safety-stop designs.

When it comes to safety in manufacturing operations, the theory tends to be 'better the devil you know'. This can sometimes spawn over-engineered solutions, based on older, but proven technologies. In electrically-driven industrial machines this has long been the case, where the essential 'emergency-stop' function has traditionally comprised an external hardwired control circuit, incorporating two series-connected interposing contactors in the power circuit.

Driving safely

The cost of contactors for a high power frequency inverter can become fairly significant, not to mention the large amount of real estate required in the cabinet. Another pitfall of the traditional design is the risk of contact-failure. That's why you need the second contactor. Without it, if the contact fails, the current will continue to flow no matter how hard you push the stop-button.

Australian Standard AS4024.1 'Safeguarding of machinery' and European standards EN954-1 and EN60204-1 'Safety of machinery' are the safety standards often applied to Australian manufacturing industry machinery. These standards require that the emergency-stop circuit immobilise and make safe all dynamic workings of the machine. This doesn't necessarily imply that the power supply needs to be cut off from the machine.

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In most cases in Australia the machines are driven by electric motors, and increasingly, these are controlled by variable speed inverters. With this in mind, the manufacturing industry initiated the development of new drive-safety technologies, integrating the safety stop function into the frequency inverter itself.

SEW-Eurodrive has been one of the pioneers in this safety concept, arming its two most popular latest-generation frequency inverters, the Movidrive 'B' and Movimot, with the integrated safety stop solution.

The integrated safety solution has all the functionality of the old hardwired system, but it's in one inclusive package. The benefits of this design fall into two groups: those to the system integrator in terms of design and installation, and operational benefits to the plant floor personnel.

Cost cutting

First and foremost, eliminating the need for two contactors by substituting the traditional safety circuit, initial component costs are reduced. Fewer components simply mean less expense. Although a drive with integrated safety features can be more costly than a standard drive, the scale tips back the other way when factoring in the additional contactors.

An OEM recently did the maths for an installation in the pharmaceutical industry. The Movidrive 'B' with integrated safety circuits worked out less costly than a standard frequency inverter with the extra contactors.

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The other area where system integrators really appreciate the difference is in installation time. The integrated safety stop solution means the engineer spends less time planning in the design stages, but it also means that the installation happens a lot quicker, without compromising the system safety characteristics.

Besides the time and cost savings, the integrated safety solution also frees up valuable space in electrical cabinets, providing an important future-proofing element: extra room could mean the difference between fitting in more drives at a later date and needing a new cabinet.

Up time up

From an operational point of view, the elimination of external components reduces maintenance needs and therefore increases reliability. The interposing contactor system was tried and tested, but it was still reliant on being kept dust free and regularly inspected. Integrating the safety solution into the frequency inverter leads to a reduction in the list of components on the plant's maintenance schedule.

Another major benefit of the frequency inverter safety solution is in the restart time after an emergency stop has been initiated. The legacy hardwired safety solution used the contactors to disable the drive by removing the power. Consequently, it also de-energised the drive's electronics. For a restart, the drive needed first to be fully re-energised, which created an inherent time lag. That restart can take a second or two, but in sectors where there's a lot of moving machinery that all needs to be restarted in sequence, this 'second-or-two' per drive accumulates to represent a significant amount of down time. In effect, an emergency stop executed by the integrated safety solution maintains the three-phase supply to the drive and only interrupts the designated safety stop control input. With the new designs on the Movidrive 'B', there's no restart delay. As a result, it doesn't really matter how often you trip the drive, the disturbance to up time doesn't compound.

Drives with integrated drive safety stop functionality are already proving popular in many Australian industrial sectors employing electric motors as the prime motive force for machines. Safety circuits incorporated into these frequency inverters must be certified in accordance with Australian and global safety standards to levels prescribed by each industry. It really is a matter of breaking from tradition and looking at the facts. It's about safe machine control instead of automatic power disconnection.